Fairphone is simply not the kind of smartphone where customers pay for additional features e.g. hard or software goodies.
In opposite fairphone offers wide openess of hard + software to let users create their own addon solution. In your case it is a simple keyboard bag for tablet or smartphones . In germany Hama is one of the cheapest:
https://de.hama.com/i/00050467/hama-otg-tablet-tasche-mit-integrierter-tastatur-displaygroesse-178-cm-7
Some people are maybe disappointed because they expect from a 500 EU/550$ smartphone “It must have this” or "It must have that for this price. " It is a pity that they misunderstand the concept and business model and feel mislead or even cheated for not so few money. If they don’t open their phone for own extensions and are not very moral inspired for the fairtrade then they maybe better not choose a fairphone.